Our present invention relates to electromagnetic moving-coil devices and, more particularly, to a so-called voice coil device with position detection.
In both linear and rotary motors, the detection of the position of a moving coil is often necessary or desirable, particularly when the motor is utilized in a closed loop control system or is a servomotor where knowledge of the position of the displaced element or load is necessary or desirable.
Position detectors utilized in the past have included incremental encoders, resolvers, Hall-effect transducers and even linear variable differential transformers (LVDT) connected to the moving coil or the actuator. In all of these systems, the detector itself is a separate element or device which is usually external of the servomotor or actuator and may take up space which could be better utilized in miniaturization systems and must be mechanically coupled to the moving part of the system.
It is the principal object of the present invention to provide an improved motor or actuator with internal position detection whereby the aforementioned disadvantages can be avoided.
Another object of the invention is to provide an improved electromagnetic moving-coil device which allows for reliable position detection in a compact system.
Still another object of this invention is to provide an electromagnetic moving-coil device or voice-coil actuator which is especially effective in miniature applications.
These objects and others which will become apparent hereinafter are attained, in accordance with the invention, by integrating an LVDT into a moving-coil or voice-coil actuator. More particularly, an electromagnetic moving-coil device can comprise:
a magnetically permeable flux-guiding outer member surrounding a space;
a moving coil within the space electrically energizable by a coil-displacement current for relative displacement of the member by interaction of magnetic fields of the member and the coil;
a flux-guiding core member surrounded by the coil, at least one of the members being provided with a permanent magnet producing a magnetic flux closing between the members and intercepted by the coil;
a high-frequency source connected to the coil for superimposing a high-frequency signal on the current; and
at least one winding on one of the members for picking up by differential transformer action a signal representing relative movement of the coil and the members.
In the system of the invention, therefore, at least one winding is provided on one of the flux-guide members, preferably the outer or sleeve member, for picking up by the differential transformer action a signal representing the relative movement of the coil and the members. Normally the sleeve or outer member and the core or near member are connected together at one end of the device and form a stationary member or stator while the moving coil is the movable part.
According to a feature of the invention, the members and the coil form a linear actuator.
The device can comprise a detector circuit connected to the winding and providing an output representing movement of the coil relative to the members.
A filter can be provided ahead of the detector circuit to remove the contribution of the PWM driver and hence interference from the energization current on the signal representing relative movement of the coil and the members.
A reference circuit can be provided to provide a base line for the output of the detector circuit and the reference circuit, for example, an automatic gain control or AGC, can be connected to a comparator with the detector circuit so that the comparator output will provide a signal having a reference point and thus representing a reference position of the coil. The current which drives the coil can be provided by a pulse width modulated or PWM driver.
The invention is particularly effective for so-called voice-coil actuators, i.e. actuators in which the coil is displaced by a current passed through it by the interaction of the magnetic field generated by that current with a flux produced by a permanent magnet and thus a coil which can be analogized to the coil of a loudspeaker.